At present, GPS receivers are very accurate and their accuracy mainly depends on numerous variables which include the ionosphere, the available satellites, the urban environment, etc. There are some factors that obstruct GPS accuracy like the following.

Physical Obstructions

The measurements of arrival time can be skewed through large masses such as buildings, mountains, trees, etc.

Atmospheric Effects

GPS devices mainly affected by solar storms, heavy storm cover, Ionospheric delays, etc.

Ephemeris

In a satellite, the orbital model could be inaccurate otherwise outdated, even though this is becoming increasingly rare.

Numerical Miscalculations

This might be a feature once the hardware of the device is not planned to conditions.

Artificial Interference

Artificial interference mainly comprises spoofs or GPS jamming devices. In open places, the accuracy of the device is high with no contiguous big buildings that can obstruct signals. So, this effect is called an urban canyon. Once a device is enclosed through large buildings, first the satellite signal can be blocked, after that bounced off a tall building, wherever it is lastly read through the device to result in faults of the satellite distance.

Fortunately, the issues faced by GPS technology have been recognized & are nearing resolution. Here, the accuracy provided by the receivers with high quality is better than 2.2-meter level accuracy within 95% of cases & superior as compared to 3-meter accuracy within 99% of cases.